The present application claims the benefit of priority under 35 U.S.C. xc2xa7119(a) from Japanese Patent Application No. 2000-181239, filed on Jun. 16, 2000, which is incorporated by reference herein.
1. Field of the Invention
The present invention relates to a permanent magnet rotor used in a rotary electric device (including a device that has a rotating rotor and a device that has a rotating coil), such as, for example, a permanent magnet electric motor and a permanent magnet generator. This invention also relates to a method of making such a rotor. This invention particularly relates to a permanent magnet rotor having an embedded magnet. The rotor provides effective utilization of the reluctance torque and reduces the leakage flux caused by the magnetic flux of the permanent magnets passing through bridges in the rotor to form a loop instead of flowing into the stator.
2. Description of the Related Art
A conventional embedded magnet type permanent magnet rotor is disclosed, for example, in Laid Open Japanese Patent Application Hei 11-262205 or in Laid Open Japanese Patent Application Hei 11-206075. The shape of one pole (in end view) of a permanent magnet rotor core 1 of an exemplary rotor 10 is illustrated in FIG. 7. As illustrated, the rotor core 1 is formed with a plurality of slits 2A, 2B, 2C in multiple layers. Each of the slits 2A, 2B, 2C has an end face in the shape of an arc. Each arc is configured such that the longitudinal ends of the arc are located in the vicinity of the outside circumferential surface of the rotor core 1 and such that the longitudinal middle portion of the arc is located radially inwardly of the end portions. Each slit extends to the opposite end of the rotor core 1 in the axial direction (perpendicular to the plane of FIG. 7) with the same cross-sectional shape as the shape of the end face shown in FIG. 7.
In order to form a permanent magnet rotor 10 with a rotor core 1 having embedded permanent magnets, the slits 2A, 2B, 2C are filled with a bond magnetic material (e.g., a plastic magnetic material) by injection molding, or the like. After filling the slits, the bond material is solidified. Alternatively, a respective permanent magnet may be machined in the shapes of each of the slits 2A, 2B, 2C, and the machined magnets are then fitted into the corresponding slits 2A, 2B, 2C.
Furthermore, with respect to the permanent magnet rotor 10 shown in FIG. 7, bridges 3 of a certain thickness are formed between the longitudinal ends of the slits 2A, 2B, 2C and the outside circumferential surface of the core 1 so that the radially outer portions (i.e., the portions on the outside circumferential surface side) and the radially inner portions (i.e., the portions on the center axis side) of the rotor core 1 with respect to the respective slits 2A, 2B, 2C will not be completely separated by the slits 2A, 2B, 2C.
However, it has been shown that in the foregoing conventional permanent magnet rotor 10 of FIG. 7, the bridges 3 formed in the rotor core 1 at the outside circumferential surface side ends of the slits 2A, 2B, 2C allow leakage flux to flow through the bridges 3, which prevents effective use of the permanent magnets. For example, FIG. 8 shows the magnetic flux generated in the permanent magnet rotor 10 and the stator pole teeth 20, in dashed lines, and illustrates the leakage flux SF flowing through the bridges 3.
In addition, the leakage flux SF in the bridges 3 causes portions of the magnetic flux density to be greater in the area of the bridges than in the surrounding areas, as illustrated in FIG. 8 by a symbol A. This causes a magnetic resistance in the magnetic paths of the q-axis magnetic flux "PHgr"q to increase, which is a factor of lowered reluctance torque.
Torque generated by the motor is written as:
T=(Pnxc3x97xcexa8axc3x97iq)+(Pnxc3x97(Ldxe2x88x92Lq)xc3x97idxc3x97iq)xe2x80x83xe2x80x83(1), 
where:
Ld is the d-axis inductance of the coil,
Lq is the q-axis inductance of the coil,
id is the d-axis component of the armature current,
iq is the q-axis component of the armature current,
xcexa8a is the interlinking flux of the armature coil due to permanent magnets, and
Pn is the number of the pairs of poles.
The direction of the d-axis is a direction of a line connecting the center of the magnet poles and the center of the rotor. The direction of the q-axis is a direction of a line passing between the poles and through the center of the rotor. That is, the direction of the q-axis is a direction at 90 degrees in electrical angle with respect to direction of the d-axis.
The first term of the expression (1) represents a torque due to the permanent magnets, and the second term a reluctance torque.
FIGS. 9 and 10 illustrate the conventional permanent rotor 10 in end view, depicting the directions of the q-axis and the d-axis. The dashed lines in FIG. 9 illustrate the directions of the q-axis magnetic flux "PHgr"q(=Lqxc3x97iq) generated by iq. The dashed lines in FIG. 10 illustrate the d-axis magnetic flux "PHgr"q(=Ldxc3x97id) generated by id.
In the embedded magnet type permanent magnet rotor, permanent magnets magnetically equivalent to air gaps are disposed in the magnetic paths of the d-axis magnetic flux "PHgr"d, so that the d-axis inductance Ld is small. On the contrary, the magnetic paths of the q-axis magnetic flux "PHgr"q pass through the rotor core 1, so that the q-axis inductance Lq is large (that is, the magnetic resistance is small). Therefore, Ld less than Lq, and appropriate currents id, iq will generate the reluctance torque (Ldxe2x88x92Lq)xc3x97idxc3x97iq.
Portions with high magnetic flux density in the bridges 3 narrow the magnetic paths of "PHgr"q and increase the magnetic resistance of the q-axis magnetic paths, which constitutes a factor of lowering the reluctance torque.
In view of the foregoing unsolved problem with the conventional permanent magnet rotor, embodiments of the present invention are directed to a permanent magnet rotor capable of providing an effective utilization of embedded magnets and the reluctance torque and to a preferred method of making the such a rotor.
One aspect of the present invention is a permanent magnet rotor with a rotor core that has permanent magnets embedded therein in slits. The longitudinal ends of the slits in which the permanent magnets are embedded are open to the outside in the circumferential surface of said rotor core. Bridges that connect the radially outer portions and the radially inner portions of the rotor core with respect to the respective slits are provided at positions inwardly of the longitudinal ends of the slits toward the longitudinal middle portions.
In preferred embodiments of this aspect of the invention, the bridges are formed, not at the longitudinal ends of the slits, but at positions inwardly of the longitudinal ends toward the middle portions of the slits. Therefore, the leakage flux is produced from either end of the bridges, so that the region of high magnetic density narrowing the magnetic paths between slits, is decreased with the help of the leakage flux. As a result, increased magnetic resistance in the magnetic paths of the q-axis magnetic flux "PHgr"q is prevented and the reluctance torque can be utilized effectively.
In preferred embodiments in accordance with this aspect of the present invention, the permanent magnets in the permanent magnet rotor are formed such that the slots are filled with bond magnet (i.e., plastic magnetic material) and the bond magnet is then solidified.
Preferably, the bond magnet is used to form the permanent magnets by injection molding, so that the permanent magnets can be embedded in the rotor core even when the shape of the slits is rather complicated. The injection molding using bond magnet may be an ordinary one in which the bond magnet is filled in the slits and solidified. On the other hand, if the bond magnet is anisotropic, the injection molding process may be an in-magnetic field injection molding process in which the bond magnet is filled in the slits and solidified in a magnetic field.
Preferably, the inside surfaces of the slits have projections or recesses formed therein, and the recesses and projections are adapted to be connected to the bond magnet when it is solidified.
Still more preferably, the projections or recesses on the inside surfaces of the slits strengthen the connection of the solidified bond magnets to the inside surfaces of the slits. Therefore, the connection between the radially outer portions and the radially inner portions of the rotor core with respect to the respective slits is strengthened through the solidified bond magnets, thereby providing a sturdier rotor core.
Also preferably, the bridges are inclined with respect to the direction of magnetization of the permanent magnets.
Still more preferably, the bridges are inclined with respect to the direction of magnetization of the permanent magnets so that the magnetic resistance in the bridges is increased without reducing the strength of the bridges. This decreases the leakage flux and improves the effectiveness of the magnets.
Another aspect of the present invention is a permanent magnet rotor with a rotor core that has permanent magnets embedded therein. The permanent magnets are formed such that bond magnet (e.g., plastic magnetic material) is filled in the slits and solidified. The inside surfaces of the slits have projections or recesses formed thereon, and the projections and recesses are adapted to be connected to the bond magnet when it is solidified.
In preferred embodiments in accordance with this aspect of the invention, the connection between the radially outer portions and the radially inner portions of the rotor core with respect to the respective slits is strengthened through the bond magnet, thereby providing a sturdier rotor core.
Another aspect of the present invention is a permanent magnet rotor with a rotor core having permanent magnets embedded in slits therein. Bridges connect the radially outer portions and the radially inner portions of the rotor core with respect to the respective slits in which the permanent magnets are embedded. The bridges are inclined with respect to the direction of magnetization of the permanent magnets.
In accordance with this aspect of the invention, the magnetic resistance in the bridges is increased without reducing the strength of the bridges. This decreases the leakage flux and improves the effectiveness of the magnets.
Another aspect of the present invention is a method of making a permanent magnet rotor with a rotor core having permanent magnets embedded therein in slits. The longitudinal ends of the slits in which the permanent magnets are embedded are open to the outside in the circumferential surface of the rotor core. Processes are provided in which treatment of the rotor core is performed with the rotor core set at a fixed angular position using the openings of the slits.
Preferably, the treatment of the rotor core is performed with the rotor core set at a fixed angular position using the openings of the slits in the circumferential surface of the rotor core. Thus, an additional device for setting the angular position of the rotor core is not needed. The treatment under the condition of the fixed angular position of the rotor core, includes, for example, filling of bond magnet into the slits, magnetization of the filled bond magnet, and the like.